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Dispersion and mortality of a population of sea scallop (Placopecten magellanicus) seeded in a tidal channel Bruce G. Hatcher, Robert E. Scheibling, Myriam A. Barbeau, Alan W. Hennigar, Lawrence H. Taylor, and Anthony J. Windust
Abstract: Ten thousand scallops (Placopecten magellanicus) from 4 to 26 mm shell height were released in 40 m2 of an 8 m deep tidal channel in Lunenburg Bay, Nova Scotia, Canada. Their survival, distribution, and potential predators were monitored during the 13 months following release in November 1990 using quadrat and video surveys. Both dispersion and mortality were rapid during the first 2 weeks, when predation by crabs and starfish was estimated to have killed about one half of the seeded population. The rate of loss decreased markedly over the following winter and spring. Dispersion of seeded animals increased again through summer and autumn, producing a final scallop density of about 2 m–2 (twice that of the natural population), covering more than 3500 m2 of seabed. This expansion was accompanied by little further mortality, and the final estimate of survivors in the expanded survey area was 40%. Scallop displacement was directional, but the mean vector did not match that of the dominant, near-bed water currents. The average growth (shell height increment) of surviving scallops was 35 mm in 13 months. Our results demonstrate the potential for ecologically viable bottom culture of the species in coastal Nova Scotia. Résumé : Dix mille pétoncles (Placopecten magellanicus) mesurant de 4 à 26 mm de hauteur de coquille ont été libérés dans une zone de 40 m2 d’un chenal à marée de 8 m de profondeur, dans la baie de Lunenburg (Nouvelle-Écosse, Canada). Nous avons surveillé leur survie, leur distribution et leurs prédateurs potentiels pendant les 13 mois qui ont suivi leur déversement en novembre 1990, en faisant des relevés sur des quadrats et en prenant des enregistrements vidéo. La dispersion et la mortalité ont été rapides pendant les 2 premières semaines, et on estime que pendant cette période la prédation par les crabes et les étoiles de mer a tué à peu près la moitié de la population ensemencée. Le taux de perte a baissé de façon notable pendant l’hiver et le printemps suivants. La dispersion des animaux a augmenté de nouveau pendant l’été et l’automne, pour aboutir à une densité d’environ 2 m–2 (deux fois celle de la population naturelle), les pétoncles ensemencés couvrant plus de 3500 m2 de fond. Cette expansion ne s’est accompagnée que d’une faible mortalité supplémentaire, et on estime à 40% le pourcentage final de survivants dans la zone élargie. Le déplacement des pétoncles était orienté, mais le vecteur principal ne correspondait pas à celui des courants dominants près du fond. La croissance moyenne (accroissement de la hauteur de la coquille) des pétoncles survivants a été de 35 mm en 13 mois. Nos résultats démontrent le potentiel d’une pectiniculture écologiquement viable sur le fond dans les eaux côtières de Nouvelle-Écosse. [Traduit par la Rédaction]
Introduction The fishery for the sea scallop (Placopecten magellanicus Gmelin, 1791) is one of Atlantic Canada’s most valuable (Sinclair et al. 1985). Depletion of coastal stocks has greatly reduced the viability of the capture fishery at many inshore sites (Caddy 1989), leading to increased interest in stock Received December 1, 1993. Accepted July 20, 1995. J11755 B.G. Hatcher. Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada. R.E. Scheibling,1 M.A. Barbeau, A.W. Hennigar, L.H. Taylor, and A.J. Windust. Department of Biology, Dalhousie University, Halifax, NS B3H 4J1, Canada. 1
Author to whom all correspondence should be addressed. e-mail:
[email protected]
enhancement and aquaculture (Naidu and Cahill 1986; Dadswell and Parsons 1991). Two common methods of scallop culture have contrasting trade-offs between yields and production costs. Suspended culture, either in pearl nets or by ear-hanging, generally results in good survival and growth rates of postsettlement juveniles (MacDonald 1986), but requires a large investment of capital and labor. Bottom culture entails release of either wild or hatchery-reared spat directly to the seabed (seeding), for harvest by conventional means (diving, dragging) following a grow-out period. The method generally trades reduced survival and growth for savings of capital and labor costs. It has been applied successfully to several species of scallops elsewhere (e.g., Ventilla 1982; Dao et al. 1994; Bull 1989; Goshima and Fujiwara 1994), but has not been used in the commercial culture of P. magellanicus, even though analysis suggests it to be the only method that offers a reasonable economic return on investment (Wildish et al. 1988; Couturier
Can. J. Fish. Aquat. Sci. 53: 38–54 (1996). Printed in Canada / Imprimé au Canada
39
Hatcher et al.
1990). Bottom seeding may be used to enhance natural stocks as well as to culture scallops in restricted leaseholds. Mortality and emigration of animals during the extended growth period of P. magellanicus (3 to 5 years from settlement to harvest) are two basic processes limiting yields from both wild and bottom-cultured populations. Suspended culture controls both types of loss by restricting animals’ movement and keeping them away from predators, such that morbidity from other causes (e.g., disease, water quality) becomes important (Dupouy 1983; Parsons and Dadswell 1992; Emerson et al. 1994). In bottom culture it is difficult to differentiate between emigration of juvenile scallops from the seeded area and mortality within it (e.g., Bull 1989). Because postsettlement movement and mortality strongly influence the distribution and abundance of scallops in natural populations (Orensanz et al. 1991), quantification of these processes in seeded populations of juvenile P. magellanicus across a range of natural environments has ecological as well as economic relevance. Predators of scallops include starfish, crabs, gastropods, and fish (e.g., Caddy 1968; Kohler and Fitzgerald 1969; Morgan et al. 1980; Volkov et al. 1983; Minchin 1992). Starfish and crabs in particular are recognized as causing significant mortality in both natural and cultured populations of P. magellanicus (Dickie and Medcof 1963; Medcof and Bourne 1964; Elner and Jamieson 1979; Dupouy 1983; Barbeau et al. 1994). Large scallops are less susceptible to predation than small ones, depending on the type of predator involved (e.g., Elner and Jamieson 1979; Barbeau and Scheibling 1994a), and animals larger than 70 mm shell height (3- to 4-year-old animals in the case of P. magellanicus) are considered able to resist attack by many predators (Brand 1991; Orensanz et al. 1991). Well endowed with sensory organs, scallops exhibit behavioral responses to predator attack such as escape swimming (Brand 1991) and migration to local refugia (Smith et al. 1988; Pohle et al. 1991). The distributions of P. magellanicus and its predators are correlated over large areas of offshore seabed (Thouzeau et al. 1991), but the behavioral and numerical responses of predator populations to local increases in juvenile scallop abundance (e.g., aggregation of starfish following seeding, Volkov et al. 1983) have not been studied and are particularly relevant to bottom culture. Juvenile P. magellanicus are competent swimmers (Caddy 1968; Dadswell and Weihs 1990; Manuel and Dadswell 1991). Swimming is a metabolically expensive activity (Thompson et al. 1980; de Zwaan et al. 1980) that may be initiated as an escape from a predator, or as an attempt to migrate from undesirable habitat. Swimming trajectories may be oriented (i.e., directional migration, Minchin 1989) or random, in which case displacement patterns may reflect the hydrodynamic regime (Moore and Marshall 1967; Posgay 1981; Caddy 1989). Tagged P. magellanicus larger than 60 mm shell height move as much as 10 km/year on Georges Bank (Posgay 1981; Melvin et al. 1985). The dispersion of smaller animals used in bottom culture may also be substantial (Carsen et al. 1995), although Parsons et al. (1992) reported net displacements of only a few metres in 3 months. Detailed, in situ measurements of spatial and temporal variation in the dispersion of seeded scallops are essential for successful management of scallop culture. As part of a major research program aimed at determining
the ecological feasibility of aquaculture and stock enhancement of Placopecten magellanicus in eastern Canada (Anonymous 1993), we followed the spatial and temporal trajectories of populations of juveniles seeded in natural benthic habitats. The results will be used to select suitable aquaculture sites, to optimize seeding and grow-out methods, and for cost–benefit analyses of production enhancement strategies. The costs of large-scale field experiments require pilot studies to optimize designs and sampling techniques. In the pilot experiment described here, we released more than 10 000 juvenile scallops in a shallow tidal channel and followed their dispersion, survivorship, and growth over a 13-month period. Specific objectives of the study were as follows: (i) to plot the trajectory of the seeded population in space and time following release in relation to the biotic and abiotic features of the environment, (ii) to estimate the losses from the population resulting from dispersion and predation in relation to the spatial and temporal patterns in abundance of various predators and their scallop prey, and (iii) to estimate the survivorship and individual growth rates in the seeded population after 1 year as measures of scallop production enhancement in a natural habitat.
Materials and methods Study site Corkum Island Channel (Fig. 1) connects two shallow basins (total area approx. 2.8 km2) with the southwest portion of Lunenburg Bay, Nova Scotia, Canada. The channel is approximately 1 km long and 0.35 km wide, with an average low tide depth of 8.0 m in the centre. The seabed grades from large cobble with kelp in the centre, through shale gravel and shell fragments, to mud and silt with sea grass at the channel margins. A shallow (≅1.0 m deep) basin (Mosher Cove) extends laterally from the southeast side of the channel. The study site (44°20.5′N, 64°18.9′W) is located in an area of uniform depth (7.5 m at low tide) and substratum (small cobbles and shell fragments mixed with mud) on the northeast side of the channel. Extensive meteorological, hydrological, and hydrodynamic measurements are available for the area (Sturley and Leal 1991; Sturley 1992). Semidiurnal tides (M2 tidal period of 12.42 h) with a mean amplitude of 1.2 m dominate flow through the channel as the shallow basins fill and empty. Along-channel current velocities (vertically averaged over the bottom 0.6 m of the water column at two locations within the study site) average 0.21 m·s–1 at 352°T (true compass bearing) on the ebb, and 0.16 m·s–1 at 233°T on the flood, with peak speeds reaching 0.6 m·s–1 on the ebb tide. An oscillation with a period of approximately 1 h is a consistent feature of the flow, with a velocity capable of halting or reversing the tidal flow. At the centre of the study site, the mean axes of the ebb and flood flows are displaced by approximately 30° to the north and west of the channel axis, respectively (Fig. 1). Daily mean seawater temperatures near the seabed at the study site ranged from –1.3 to 16.4°C during the 13 months. Semidiurnal temperature variations were often substantial, exceeding 4°C during the autumn and spring. Salinity ranged from 29.1 to 31.4 parts per thousand (ppt) during this period,
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Can. J. Fish. Aquat. Sci. Vol. 53, 1996
Fig. 1. Chart of the study site in Lunenburg Bay, Nova Scotia, showing the channel east of Corkum Island, the layout of the survey area, and the 40-m2 release area in which scallops were seeded in November of 1990. The along-channel orientation of the main sampling transect, the mean tidal current vectors, and the mean vector of movement of the seeded scallop population (156°) during the 13-month experiment are indicated with arrows (true compass bearings).
with semidiurnal variations of approximately 1.0 ppt during spring tides. Preseeding survey The distribution and abundance of scallops and potential predators were measured using self-contained underwater breathing apparatus (SCUBA) during the day prior to seeding (November 19, 1990), in four 25 × 1.0 m belt transects laid parallel to the main axis of the channel. Two transects ran through the centre of the area to be seeded with scallops, and the other two were located 10 m towards the eastern shore. All scallops and scallop valves in three size-classes (5–30, 30 – 100, and >100 mm as measured against a diver-held scale) were counted in contiguous 0.25-m2 quadrats along each transect, as were all potential predators of scallops (e.g., starfish, crabs, snails, fish). Scallop handling and seeding Juvenile Placopecten magellanicus, ranging from 4 to 26 mm shell height, were purchased from a commercial hatchery (Fisheries Resource Development Ltd.) on November 19, 1990. The animals were the progeny of a single, captive
spawning of adults collected from Mahone Bay (adjacent to Lunenburg Bay). The scallops were transported in coolers directly from holding facilities in Mahone Bay to the study site. There they were counted, sorted into groups of 400, and suspended in pearl nets in the channel near the seeding site. A sample of 1022 animals (10% of the seeded population) was haphazardly selected from the total population during this process to serve as tagged tracers. They were marked by painting a spot of white nail polish on the dorsal valve and subsequently remixed with the entire seed population just prior to release. A further four groups of 60 scallops each (including 6 marked individuals) were selected haphazardly from the seed population to serve as controls for postseeding mortality owing to factors other than predation (e.g., handling, disease). They were placed in 0.38 × 0.38 m pearl nets with a nominal (stretched) 5 mm mesh size, and suspended 2.5 m off the seabed in the study site at the time of seeding. On November 20, 1990, 10 220 scallops (including the marked sample) were released by divers in a demarcated 10 × 4 m area by pouring the animals out of buckets from 0.5 m above the seabed. Seeding occurred at dusk (between 16:30
41
Hatcher et al. Table 1. Characteristics of the survey area and sampling of a seeded scallop population in the channel east of Corkum Island, Lunenburg Bay, Nova Scotia, prior to and in the year following the release of 10 220 juvenile Placopecten magellanicus onto the seabed. Survey dates 20 Nov. 1990 (preseed survey) 21 Nov. 1990 (day 1) 26–27 Nov. 1990 (day 6) 6–7 Dec. 1990 (day 16) 30–31 Jan. 1991 (day 71) 20 Feb. 1991 (far-field survey) 28 Feb. 1991 (far-field survey) 26–27 Mar. 1991 (day 128) 15–16 May 1991 (day 177) 25–26 July 1991 (day 248) 19 Dec. 1991 (day 394)
Days after seeding
Survey area (m2)
–0.5
Total sample area
No. of quadrats in sample
Sample effort (h)
Sea temp. (oC)
Counts of seeded scallops
m2
%
na
100
na
400a
8.3
7.0
na
na
+0.8
130
26
20
105
5.4
6.5
2147
35
6–7
590
89
15
356
13.1
6.5
1491
150
16–17
1250
179
14
716
14.5
4.5
782
129
71–72
520
120
23
481
11.4
1.0
555
202
92
3930
864
22
3456a
3.0
–0.5
3
nd
100
4640
1052
23
4208a
3.5
0.0
52
nd
128–129
1000
193
19
772
16.0
5.0
847
171
177–178
1140
254
22
1017
11.9
7.0
716
370
248–249
3640
470
13
1879
18.0
13.0
643
471
394
7130
571
8
2284b
9.3
4.5
330
nd
Live
Dead
Note: Sample results are given in terms of the effort expended in man-hours underwater to count live and dead scallops in quadrats covering a proportional sample area of the survey area on each date. na, not applicable; nd, no data. a Linear or circular transect surveys. b Video transect survey, expressed in terms of 0.25-m2 quadrats.
and 17:00 Atlantic standard time), just after slack low water. During this period, depth-averaged currents in the 0.6 m above the seabed ranged from 0.14 to 0.22 m·s–1 on 260 to 240°T. Sea temperature at the time was 6.5°C, salinity 31.2 ppt, and horizontal visibility 4.5 m. Postseeding monitoring The 40-m2 release area was centered on a 50-m graduated (0.5-m interval) lead line placed parallel to the central axis of the channel (Fig. 1). The survey area on either side of the transect was quantitatively sampled by SCUBA divers using 0.5 × 0.5 m (0.25 m2) quadrats on the day of seeding immediately prior to the release of the juvenile scallops, and at 16 h and 6, 16, 71, 128, 177, and 248 days after seeding (Table 1). In each survey, the numbers of live scallops, intact and broken shells of seeded scallops, and predators were counted in contiguous series of 0.25-m2 quadrats extending perpendicularly (i.e., across channel) from both sides of randomly selected points on the central transect line. The quadrats were sampled to the edge of a predesignated survey area, or until at least three successive samples yielded zero counts of live and dead seeded scallops. Thus, the area sampled expanded as the seeded population of scallops dispersed, with marked increases in the survey area on days 6, 16, and 248 (the larger surveys took 2 days to complete, Table 1). Far-field surveys were conducted during February (days 92
and 100, Table 1) to determine if substantial numbers of seeded scallops had moved beyond the operationally defined survey area. Semicircular search patterns were extended to 50 m radius from the northeast (towards the open bay) and southwest ends of the central transect line (Fig. 1). Another circular search was done 50 m southeast of the survey area, in Mosher Cove. Live seeded scallops only were counted in 1 m wide radii at 5-m intervals by divers holding a tape secured to a mooring. On the last sample date (day 394) the central transect line was extended to 125 m along 215°T, and an area exceeding 7500 m2 was surveyed with a diver-held Hi 8-mm video camera (Sony CCD-V101) in an underwater housing (Amphibico Ltd., Montréal). Moveable 50-m transect lines (graduated at 0.5-m intervals) were centered perpendicular to the central transect at randomly selected points along it. A 0.5 m wide strip of seabed was imaged along the moveable transects, using a graduated focal-distance rod to keep the camera 0.6 m above the seabed and centered on the transect line. Calibration quadrats were embedded within the video transects, and counted by a diver immediately following the passage of the camera. The video tapes were analyzed (Sony EV-5900 VCR, Sony KV-20EXR20 color monitor) in 0.5-m sections (equivalent to 0.25-m2 quadrat samples), and the same data recorded as for quadrat surveys except that the remains of dead scallops were not counted.
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The area sampled by quadrats within a given survey ranged from about 13 to 23% of the total area surveyed, while the area sampled by video in the last survey was about 8% (Table 1). Unmarked scallops belonging to the seeded population were readily distinguished from those of the natural population on the basis of their uniform, small size and the distinct patterns of radial color bands on their dorsal valves. Seeded animals became more difficult to identify as they diverged in size because of differential growth, and as they became fouled with epibionts. Calculated densities of living and dead seeded scallops, wild scallops, and predators were stratified according to location within the survey area. The strata were as follows: the release area (40 m2), a predesignated 25 × 20 m area (500 m2) centered on the release area, and an extended border area defined operationally as described above. Total abundances of each class of organisms within the entire survey area on a given date were estimated by integrating sample densities across all strata. Distance-weighted, least-squares interpolation algorithms (McLain 1972, as implemented by Wilkinson 1991) were used to plot density contours of scallop and predator abundances for each survey date. Vector analysis was used to calculate the distance and bearing from the centre point of the release area to the location of each sampled scallop (±0.25 m and ±3° accuracy) on each survey date. Radial triangulation to the outermost scallops located in each survey was used to calculate the actual area covered by the seeded population. Population dispersion rates (KA, apparent horizontal diffusivities) were calculated from the mean square radial displacement of scallops from the centre of the release area on subsequent sampling dates, and plotted against the time since seeding (Okubo 1971). The survival of scallops in pearl nets was monitored on each survey date until day 177. The low frequency of sampling beyond this date meant that the nets could not be adequately cleared of fouling organisms, and the observations were discontinued. Haphazardly selected subsamples (n = 183) of the seeded and wild population were collected on the day before seeding and on day 380 (n = 230) for size-frequency analyses. Shell height was measured with vernier calipers to the nearest 0.1 mm before the animals were returned to the population. Independent mortality assays Rates of mortality of juvenile scallops were measured before and after seeding using arrays of animals tethered to the seabed within the survey area. Pilot studies in the laboratory demonstrated that the tethers allowed a limited escape swimming response to attack by starfish. Scallops from the same population as that seeded were attached to numbered stainless steel stakes with 15-cm lengths of 2.5-kg test monofilament fishing line glued to the dorsal valve with rapid-set epoxy glue. Groups of 25 tethered animals in the size range 7–30 mm were evenly spaced in 3 m diameter circular arrays (i.e., about 30 cm apart) by pressing the stakes into the substratum around fixed markers at three points 25 m apart along the central transect. Tethered scallops were inspected daily by divers for the first 2 days, and then at longer intervals until the number surviving decreased to about 20% of the initial number. The causes of mortality could be inferred from the remains on the tethers. Death owing to handling stress left tissue within
Can. J. Fish. Aquat. Sci. Vol. 53, 1996
gaping valves in the first day; starfish predation left undamaged, empty valves; crab predation left fragments of shell; and fish predation (or other forms of mechanical separation) left a broken tether or one with glue only. To control for loss of scallops from tethers as a result of factors other than predation, eight scallops were tethered inside a 1 m diameter by 0.5 m high cage (15-mm mesh aperture) near the central array. The survival of this control group was monitored at the same time as the other arrays. All data were analyzed using the SYSTAT and SYGRAPH computer package (Wilkinson 1991). Standard deviations are given after means, and statistical tests are reported at the p = 0.05 level of significance, unless otherwise noted. Circular statistics (Batschelet 1981) were used to test whether the distribution of the scallop displacement angles differed significantly from a random one, and from the tidal and residual current vectors for each survey independently.
Results Preseeding conditions The natural population of Placopecten magellanicus in the survey area prior to seeding occurred at an average density of 0.40 ± 0.11 m–2 (Fig. 2). It had a broad size distribution (approx. 6–160 mm shell height), in which about 16% were juveniles in the size range of the seeded population (i.e., 0.1), demonstrating that diffusion was non-Fickian (Okubo 1971). KA was poorly described by a quadratic regression (Fig. 4b), but was significantly correlated with mean ambient water temperature between survey dates over the course of the experiment (r = 0.757, P < 0.01, n = 8). Dispersion rates were low in winter and spring, when average sea temperatures were below 5°C. The spatial distribution of the seeded population during the first 8 months showed a consistent orientation towards the southern and eastern portions of the survey area (Fig. 5). The distribution of scallop displacement vectors differed significantly from random in all surveys (Rayleigh’s test, P < 0.01), and the mean displacement angles differed significantly between the eight surveys (Watson–William’s test, F[7,750] = 17.81, P < 0.001). During the last 5 months of the study a substantial westerly expansion of the distribution towards the centre of the channel occurred. The pattern of scallop distribution did not correspond well to the tidally dominated pattern of water movement immediately above the seabed at the release site (Figs. 1 and 5). The mean angle of net scallop movement from the release point varied from 137 to 176°T between surveys over the duration of the study. These mean angles differed from the main tidal current axes (ebb = 352°, flood = 233°; Rayleigh’s tests, P < 0.001) and the calculated Eulerian residual current (330°T). The abundance of dead scallops within the survey areas
44 Fig. 4. Dispersion characteristics of a population of juvenile scallops (Placopecten magellanicus) during 13 months following release in the centre of the survey area near Corkum Island, Nova Scotia. (a) The mean displacement distance of seeded scallops from the centre of the release area (open circles) and the total area in which live scallops were found (solid circles) are shown for each survey date. Errors bars represent one half of the standard error of the mean. (b) The variance of the individual scallop displacements (open circles) and the apparent horizontal diffusivity of the seeded scallop population (solid circles) are plotted as a function of time since release. Exponential and quadratic curves are fitted (maximum likelihood) to the data. Sample sizes range from 2147 to 330 scallops.
showed no consistent pattern during the study. The spatially averaged density of dead animals varied significantly (ANOVA, F[6,5319] = 8.373, P < 0.001) over the 248-day period in which it was measured, reaching a low of 0.72 m–2 on day 16, peaking at 1.69 m–2 6 days after seeding, and averaging 1.00 m–2 after 248 days. The estimated total numbers of dead scallops at each survey ranged from 106 on day 1 to 3640 on day 248 (Fig. 6). Their spatial distribution showed a consistent pattern of constant numbers and high density (4–8 m–2) in the release area from day 6 to day 177, and low densities ( 0.05). Scallop mortality Most of the mortality in the four tethering experiments was attributable to starfish predation, as 60–80% of the remains were clean, undamaged valves (Fig. 9). Only about 1% of intact valves had flesh within them on the first day after release, indicating low nonpredatory mortality. Between 0.5 and 8% of the remains were broken valve fragments indicative of crab predation. The remainder of the losses left no remains, and were attributed to removal of scallops by crabs, fish, or rarer predators, because detachment of scallops from tethers in the cage control was 0.1). The estimated time to 80% mortality in these experiments was between 10 and 20 days, and the proportion of the mortality resulting from crab predation increased following seeding. Mortality during the winter and early spring (70 to 157 days after seeding) was significantly lower (ANOVA, Tukey’s test, P < 0.005), with a time to 80% mortality of over 60 days. By late spring mortality had again increased such that the survivorship curve overlapped those of the autumn experiments, with a time to 80% mortality of about 15 days. The rate of predation loss from tethering arrays during the first 10 days of each experiment was correlated with the average sea temperature during that period (r = 0.960, P < 0.05, n = 4). Scallop mortality rates derived from the tethering experiments were markedly higher than those derived from change in the estimated size of the seeded population. The initial slope of the population survivorship curve (Fig. 7) corresponded to a loss rate of about 3.2%/day over the first 16 days after seeding, which was less than half that of 7.8%/day measured in the concurrent tethering experiment (Fig. 9). More significantly, the survivorship curve approached an asymptote between 24 and 49% surviving after about 3 months, while the extended tethering curves continued to decline below 20%. These contrasts are conservative, because the population survivorship curve does not include animals that had moved outside the survey areas. The total mortality of seeded scallops estimated from sampled densities of remains was about 1% on day 1, reached about 10% on day 6 and remained at this level until day 128, and then increased to reach 36% of the seeded population by day 248 (Fig. 6). These mortalities ranged from 10% (day 71) to 70% (day 248) of the losses estimated from the densities of live animals (Figs. 5 and 7) and were much lower than the independent mortality rate measurements (Fig. 9). There was no mortality of scallops kept in pearl nets at the study site during the first 177 days of the experiment. Scallop growth The size-frequency distributions of the seeded scallop population at the time of seeding and on day 394 approximated nor-
Fig. 7. Temporal variation in the survivorship of a population of juvenile scallops (Placopecten magellanicus) released on the seabed in a channel near Corkum Island, Nova Scotia, on 20 November, 1995. The least-squares negative exponential curve has a slope of –0.0014 (r2 = 0.160, n = 9).
mal distributions with nonoverlapping tails (Fig. 10). The hand-measured sample taken on day 380 and the video sample from day 394 yielded similar means (t = 0.312, P > 0.5) and variance (F = 1.03, P[two-tailed] > 0.2) distribution statistics, so the larger video sample was used to characterize the final population. Over the course of the experiment the range of shell heights diverged from an initial spread of 4–26 mm (22 mm) to a final range of 36–66 mm (30 mm). The means did not diverge markedly from the modes of samples in either the initial or final surveys, which allowed prediction of the average shell growth of individuals within the population over the duration of the study. The size increment of 34.9 mm over the 13-month growth period corresponds to an average growth rate of 0.09 mm·day–1.
Discussion Interpretation of the trajectory of the seeded population Accurate description of the trajectory of the seeded scallop population depends on the ability of the sampling scheme to estimate change in abundance through space and time. Rates of survivorship can only be inferred if the entire population is sampled representatively. Sampling to the edges of the population’s distribution is required to avoid scoring emigration as mortality, although it is not essential for good estimates of rates of horizontal dispersion. Our sampling probably failed to encompass the outermost distribution of the population beyond day 6, thereby confounding the effects of dispersion and mortality in estimating true population size. The relative importance of these processes in determining observed changes in scallop density cannot be unequivocally resolved from the abundance data alone. The problem is common to all seeding experiments in natural habitats (e.g., Morgan et al. 1980; Tegner and Butler 1985; Bull 1989; Dao et al. 1994), and its resolution requires advances in techniques for surveying low densities of animals over extensive areas of seabed. The four surveys on days 16 to 177 (inclusive) underestimated the actual population by that proportion which had dispersed beyond the limits of the 50 × 25 m survey area (Fig. 5). The sample on day 71 was biased by the effects of cold water, strong currents, and poor visibility on divers’
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Can. J. Fish. Aquat. Sci. Vol. 53, 1996
Fig. 8. Spatial distribution of starfish (Asterias forbesi and Asterias vulgaris) during 13 months following release of a population of juvenile scallops on the seabed in a channel near Corkum Island, Nova Scotia. The total number of animals estimated from sampling within the survey area (and the calculated mean density throughout that area) is given above each plot. Other survey details as for Fig. 5.
49
Hatcher et al. Fig. 9. Survivorship (a) and evidence of mortality (b) of groups of juvenile scallops (Placopecten magellanicus) tethered in the seabed survey area of a channel near Corkum Island, Nova Scotia, prior to and during seeding on November 20, 1995. Cluckers are clean, undamaged, but empty valves. Experiment 1, preseeding period, 23 Oct. – 15 Nov. 1990; experiment 2, postseeding period, 26 Nov. – 6 Dec. 1990; experiment 3, postseeding period, 29 Jan. – 26 Apr. 1991; experiment 4, postseeding period 9 May – 18 June 1991. Error bars represent the standard error of the sample means. Samples consisted of three replicate arrays with 25 tethered scallops in each.
ability to discern the seeded scallops. The count in the next survey (day 128) suggests that this poorest sample underestimated the population by about 20%, while the few scallops discovered in subsequent far-field surveys demonstrate that only a small proportion (
